When a wild trout gets to a certain size the only high protein food source available to them in sufficient size and quantity especially from the less fertile lochs is to eat their own species.   These cannibals are top of the food chain; few in number and because of the size of food they eat do not need to eat very often and consequently are rarely caught. WH Lawrie says in his  book 'Big Trout'  that almost without exception these big trout are mainly nocturnal feeders-though this is only partly true because they still show from time to time in the day.   
Catching these large cannibal’s  on the fly by design is an area of fly fishing that has great scope for tactical development though the difficulty of trying to catch these fish by design on the fly should not be underestimated
Some of the difficulties are 
-   the kind of waters these cannibals live in are usually full of small trout and so generally not fished seriously by those in search of big fish
-   They exist in small numbers being top of the food chain and do not need to feed often because of the size of the food they eat. 
 -   To cast fly’s the size of small trout is not easy on the 6/7 wt. rods most commonly used by loch anglers and investing in  i.e. 8-9 wt. rods needed takes the pursuit into another dimension of commitment and seriousness that is likely to be a picture of lots of effort for very little reward- however the challenge is there
-   It's very difficult to resist the joy of catching lots of smaller trout that are more numerous  and put on large trout imitating fly’s where the odds are stacked heavily against the angler